Laser-induced breakdown spectroscopy (LIBS) is an elemental analysis technique that detects the atomic emissions from a plasma. The plasma is formed by focusing a high energy laser pulse on a sample. LIBS provides for real time and in-situ measuring of the elemental composition of samples of interest. In a LIBS system, a short pulse (˜ns) laser beam is tightly focused on a sample.
A typical laser used is a pulsed (50 to 100 mJ), high power, near-IR laser, such as Nd:YAG, Nd:YVO4, and Nd:YLF lasers. The high intensity laser spot that is formed heats, vaporizes, atomizes and ionizes the constituent elements of the sample within a small volume (<0.1 mm3) by forming a high temperature plasma or spark. Since the temperature in the plasma is typically around 10,000 K, the ablated atoms are ultimately electronically excited.
As the ions relax, they emit radiation at characteristic wavelengths. The characteristic spectrum typically ranges from the ultraviolet to near-infrared. The intensities of the emission lines emitted when the plasma decays are analyzed using a spectrometer. This signal radiation is weak in comparison to the plasma-inducing laser pulse, so a highly sensitive, gated detector is commonly used with a filter to eliminate the laser light. The detector is typically either a charge-coupled-device array, or a camera. These intensities can provide quantitative and qualitative information regarding the sample.
Use of LIBS techniques has grown due to the development of new laser sources and instrumentation. In addition, low-cost spectrometers with high spectral coverage and resolution have opened up new applications. Other advantages afforded by LIBS include the lack of any required sample preparation, results are obtained in near real time, and remote analysis is possible. Optical endoscopes or fiber optic cables provide in situ analysis of materials in remote or hazardous environments.
There are still many problems associated with LIBS systems. Whereas qualitative analysis has been widely demonstrated for a range of samples, quantitative measurements are more affected by the substrate properties. The temperature reached after the laser pulse and the electron density are fundamental parameters of plasma formation processes and dynamics and are directly dependant on the laser/sample interaction at a given wavelength. One important parameter relating to plasma dynamics is the decay time, which is strictly related the selection of the detection window chosen in which local thermodynamic equilibrium can be assumed to hold. Another difficulty is plasma expansion, which is not well understood. To breakdown the chemical elements of a sample, the surface area ablated by the laser pulse and the shockwave must be small. The laser spot intensity focused on the sample must exceed a certain threshold to form a plasma. Tight and optimum focusing is crucial in a LIBS system for reaching the intensity threshold required. At the same time, it is desirable to keep the total laser power as low as possible while still exceeding the intensity threshold. Using a laser of lower power not only decreases system cost but also reduces the possibility of damaging the sample.
In prior-art LIBS systems, a visible guide light beam from a separated light source such as low-power HeNe laser is employed for laser focusing and target identification. Both focusing and target identification require this visible guide light being well aligned with the high-power near-IR laser beam. Obviously, such an arrangement complicates LIBS systems since it requires an extra light source, more optics and cumbersome alignment.
Also, the typical LIBS system, especially one with portability requires frequent wavelength calibration due to mechanical factors such as vibration and heating.
One second order nonlinear optical process is that of second harmonic generation The second harmonic is generated by passing a beam with angular frequency ω,=Zπv through a crystal having a nonzero value of Xz such that the output beam emerging from the crystal contains both the frequency of the fundamental wave and twice the frequency of the fundamental wave. From the photon perspective, the energy levels of the photons of the fundamental frequency combine to produce twice the energy level. These levels do not accrue population since they are not eigenstates of the material. Instead, the photons are destroyed and simultaneously recreated as a single photon of twice the frequency.